Nikon D90 Imatest Results

We routinely use Norman Koren's excellent "Imatest" analysis program for quantitative, thoroughly objective analysis of digicam test images. I highly recommend it to our technically-oriented readers, as it's far and away the best, most comprehensive analysis program I've found to date.

My comments below are just brief observations of what we see in the Imatest results. A full discussion of all the data Imatest produces is really beyond the scope of this review: Visit the Imatest web site for a full discussion of what the program measures, how it performs its computations, and how to interpret its output.

The Nikon D90 showed very good color accuracy overall. Hue accuracy was also quite good, with most of the hue shift occurring in the cyans, sky blues, and reds. Average saturation was 103.8% (oversaturated by only 3.8%, mostly in the blues, reds and dark greens). Average "delta-C" color error was an unusually low 4.21 after correction for saturation, which is excellent. All in all, a very good color response for an SLR. Mouse over the links below the illustration above to compare results with competing models.

Using the Adobe RGB color space (which provides a much wider gamut, or range of colors that can be expressed), the Nikon D90 delivers more highly saturated color, with an average saturation of 109.7% and average saturation-corrected hue error of 4.49 "delta-C" units, which in this case is slightly more accurate than most of the competition here. Again, mouse over the links below the illustration above to compare results with competing models.

Color Analysis

This image shows how the Nikon D90 actually rendered the colors of the MacBeth chart, compared to a numerically ideal treatment. In each color swatch, the outer perimeter shows the color as actually captured by the camera, the inner square shows the numerically ideal color, after correcting for the luminance of the photographed chart (as determined by a second-order curve fit to the values of the gray swatches), and the small rectangle inside the inner square shows the numerically ideal color, without any luminance correction. This image shows the D90's very good hue accuracy, as well as a contrast curve that results in moderate overexposure of some highly saturated colors. (We've seen this in most cameras, increasing the "punch" of images by lightening bright colors a little, while actually keeping both the hue and saturation level very close to technically accurate values.)

Noise Analysis

There's a lot in this particular graph, a lot more than we have room to go into here. (This set of plots has also changed a little in the more recent versions of Imatest. Some of the plots that were shown here previously are now shown in other Imatest output. Since we largely focus on the Noise Spectrum plot, we'll only show the graphic above, which includes that plot.)

In comparing these graphs with those from competing cameras, I've found that the Noise Spectrum graph at lower right is the most important. Cameras that manage to shift their noise spectrum to higher frequencies have much finer-grained noise structures, making their noise less visually objectionable. In the graph above, this would show up as a noise spectrum curve that remained higher on the right side, representing higher noise frequencies. The champion at this was (and still is) the Canon EOS-1Ds Mark II, which produced remarkably fine-grained image noise, even at very high ISOs.

ABOVE, we see the results at ISO 100, which is as a low extension of the normal ISO range for the D90. The luminance curve is reasonably flat except for the lowest frequencies, however the red and blue color channels exhibit higher noise values, especially at lower frequencies. While the overall noise levels are very low, the dominance of low frequencies in the red and blue channels point to low-level blotchiness.

Here we see a slight improvement in overall noise levels at the D90's base ISO?of 200, however the graph is still skewed to the left, indicating a coarser "grain" in the small amount of noise that is present. The blue and red channels follow the green and luminance channels more closely than at ISO 100, but still show a bit more noise energy, especially at lower frequencies.

Above is the same set of noise data at ISO 3,200. Here, we can see the overall noise energy has gone up somewhat (the area under the curves is larger), and the Noise Spectrum graph is still shifted toward the left-hand, lower-frequency side. This time, the red channel has slightly more noise energy at lowest frequencies, though the channels are more closely matched than at ISOs 100 and 200, an effect of the D90's default high ISO noise reduction.

Here's the same set of noise data at ISO 6,400. Very similar to ISO 3,200, with a slightly larger area under the curve, indicating higher noise levels overall, but a similar grain pattern. (Good that there isn't a big shift toward low frequencies.)

This chart compares the Nikon D90's noise performance over a range of ISOs against that of other competing cameras, using default settings. While we continue to show noise plots of this sort because readers ask for them, we each time point out that the noise magnitude is only a small part of the story, the grain pattern being much more important. In the case of the Nikon D90, the magnitude of the image noise starts out slightly below most of the competition at ISO 100, but rises a bit faster than most up to ISO?800. Comparing D90 to D80 shots at ISO?800 confirms this, as the D80 shots look cleaner, but with less detail since its high ISO?NR starts more aggressively at ISO?800. At ISO 1,600, the D90's default high ISO?noise reduction kicks in more aggressively, keeping noise magnitude almost level out to 3,200, after which is climbs again at a higher rate to ISO 6400. (The D90 is the only camera in this group capable of ISO 6400). Keep in mind these are at default settings, so the shape and positions of the curves will be influenced by your settings.

Nikon D90 Dynamic Range Analysis

A key parameter in a digital camera is its Dynamic Range, the range of brightness that can be faithfully recorded. At the upper end of the tonal scale, dynamic range is dictated by the point at which the RGB data "saturates" at values of 255, 255, 255. At the lower end of the tonal scale, dynamic range is determined by the point at which there ceases to be any useful difference between adjacent tonal steps. Note the use of the qualifier "useful" in there: While it's tempting to evaluate dynamic range as the maximum number of tonal steps that can be discerned at all, that measure of dynamic range has very little relevance to real-world photography. What we care about as photographers is how much detail we can pull out of the shadows before image noise becomes too objectionable. This, of course, is a very subjective matter, and will vary with the application and even the subject matter in question. (Noise will be much more visible in subjects with large areas of flat tints and subtle shading than it would in subjects with strong, highly contrasting surface texture.)

What makes most sense then, is to specify useful dynamic range in terms of the point at which image noise reaches some agreed-upon threshold. To this end, Imatest computes a number of different dynamic range measurements, based on a variety of image noise thresholds. The noise thresholds are specified in terms of f-stops of equivalent luminance variation in the final image file, and dynamic range is computed for noise thresholds of 1.0 (low image quality), 0.5 (medium image quality), 0.25 (medium-high image quality) and 0.1 (high image quality). For most photographers and most applications, the noise thresholds of 0.5 and 0.25 f-stops are probably the most relevant to the production of acceptable-quality finished images, but many noise-sensitive shooters will insist on the 0.1 f-stop limit for their most critical work.

The image below shows the test results from Imatest for an in-camera JPEG file from the Nikon D90 with a nominally-exposed density step target (Stouffer 4110), and the D90's settings at their default positions, where Active D-Lighting is set to Off.

These are very good results for in-camera JPEGs, among the top performers in the high quality category (7.77 f-stops). The tone curve shows excellent gradation in highlights but tails-off very abruptly in the shadow end. This can be seen when closely inspecting shots captured by the D90, where detail is held very nicely in strong highlights, but somewhat less so in the deepest shadows.

Not surprisingly, the results with Active D-Lighting set to Low are a bit lower at the highest quality level, because shadow noise has increased slightly. The total dynamic range figure (irrespective of noise levels) has increased slightly from 9.05 to 9.77 f-stops.

Very similar to the Low setting, the Active D-Lighting Normal setting had slightly better results, with dynamic range at the highest quality increasing slightly to 7.75 f-stops, and the total dynamic range to 9.89 f-stops.

It's interesting that the Extra High setting actually reduced both dynamic range figures. This is because it's really just redistributing the available tonal scale, which involves boosting brightness levels in the shadows pretty substantially. This also boosts the noise, so the useful dynamic range is reduced.

Processing the D90's RAW (NEF) files through Adobe Camera Raw (ACR) version 4.6 (beta) improved dynamic range by almost 2 stops at the best quality level, and by more than 3 stops overall. ACR's automatic settings were used., tweaking the sliders manually may result in slightly better results, but these numbers are already excellent. The response curve has an almost ideal "S" shape to it, where the shadow end of the curve trails off much more gradually than in-camera JPEGs. (Although the highlight side ends a bit more abruptly than we'd prefer.) Worth noting here is that ACR's default noise reduction settings also reduced the pixel noise relative to the levels in the in-camera JPEG. Also important to note is that the degree of manipulation performed by ACR here would likely result in color errors in strong highlights in real-world photographs. (The best performance you're likely to see in a real-world environment will likely be somewhere between the in-camera results and those from Adobe Camera Raw.)

Dynamic Range, the bottom line:

The net result was that the D90 JPEGs and ACR processed RAW files came in above most other DSLR models we've tested recently at the highest quality level, including the D300.

To get some perspective, here's a summary of the Nikon D90's dynamic range performance, and how it compares to other digital SLRs that we also have Imatest dynamic range data for. (Results are arranged in order of decreasing dynamic range at the "High" quality level.):

The results shown in the table are interesting. One of the first things that struck me when I initially looked at test data for a wide range of d-SLRs, was that here again, purely analytical measurements don't necessarily correlate all that well with actual photographic experience. There's no question that the Fuji S3 Pro deserves its place atop the list, as its unique "SR" technology does indeed deliver a very obvious improvement in tonal range in the highlight portion of the tonal scale. I was surprised to see the analytical results place the Olympus E-300 as highly as they did, given that our sense of that camera's images was that they were in fact noisier than those of many other d-SLRs that we looked at. In the other direction, I was quite surprised to see the Nikon D2x place as low on the listings as it did, given that we found that camera's shadow detail to be little short of amazing.

One thing that's going on here though, is that we tested each camera at its lowest (base) ISO setting, which should produce best-case noise levels. This is in fact what many photographers will be most interested in, but it does perhaps place some of the Nikons (like the D40 and D90) at a disadvantage, as their base ISO setting is 200, as compared to the ISO 100 settings available on most other models.

All that said, the Nikon D90 delivers excellent dynamic range performance, clearly one of the leading cameras currently on the market in this regard.

Nikon D90 Resolution Chart Test Results

The chart above shows consolidated results from spatial frequency response measurements in both the horizontal and vertical axes. The "MTF 50" numbers tend to correlate best with visual perceptions of sharpness, so those are what I focus on here. The uncorrected resolution figures are 1,329 line widths per picture height in the horizontal direction (corresponding to the vertically-oriented edge), and 1,400 lines along the vertical axis (corresponding to the horizontally-oriented edge), for a combined average of 1,365 LW/PH. Correcting to a "standardized" sharpening with a one-pixel radius increased both vertical and horizontal resolution significantly, resulting in an average of 2,075 LW/PH, somewhat below what we'd expect from a 12-megapixel DSLR.

To see what's going on, refer to the plots below, which show the actual edge profiles for both horizontal and vertical edges, in both their original and corrected forms. Here, you can see that the D90's default in-camera sharpening is fairly conservative (only a slight bump at the top ends, and no noticeable bump at the bottom ends of the black edge profile curves). Imatest reports that the horizontal direction (vertical edge) is "undersharpened" by 28.9% while the vertical direction (horizontal edge) is undersharpened by 26.3%. Professionals and serious amateurs prefer this to oversharpening, and the D90's images respond very well to the use of strong/tight sharpening post-exposure in Photoshop or other image editors. (That said, you should be able to extract still more fine detail if you begin with a RAW file, rather than a JPEG.)
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